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Author Topic: How do I explain quantum spin for a lay person?  (Read 1853 times)

Offline chris

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How do I explain quantum spin for a lay person?
« on: 25/03/2016 16:06:43 »
I was asked today at a Q and A session by a member of the public to please explain what physicists mean when they talk about quantum spin.

I'd be really interested in how everyone here would have approached this please?


 

Offline agyejy

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Re: How do I explain quantum spin for a lay person?
« Reply #1 on: 25/03/2016 16:35:17 »
It's probably best to open with the fact that spin acts a lot like angular momentum. Basically describing how thinking of an electron as a spinning sphere makes a certain amount of sense but ultimately the model fails to predict reality. The speed at which an electron would have to spin given its maximum size (assuming it isn't a point particle) to produce the observed properties would be faster than the speed of light so clearly the spinning sphere model has some issues. Then you could move on to talking about something seemingly unrelated but probably familiar; the polarization of light. From there you link the polarization of light to the spin of the photon and then you talk about how all quantum particles have wave characteristics and how you can sort of think of spin as a measure of the polarization of the waves. Then you finish up talking about how all these are just models of reality and that actual reality may be so different from our everyday experiences that we'll never fully understand what quantum spin is and in the end might have to settle for imperfect analogies.
 

Offline jeffreyH

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Re: How do I explain quantum spin for a lay person?
« Reply #2 on: 25/03/2016 16:47:34 »
If we think of symmetry we can rotate a sphere along an axis in any orientation. It will always appear the same after rotation. In the case of particles a spin of 360 degrees does not return it to a state that is identical to its original state. This only happens after a 720 degree rotation. Much like a path around a Mobius strip.
 

Offline alancalverd

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Re: How do I explain quantum spin for a lay person?
« Reply #3 on: 25/03/2016 20:08:15 »
The object of quantum mechanics is to build an explanatory and predictive mathematical model of very small things.

We can build a predictive model of an atom by allowing its constituent parts to have quantised energy states. We can then explain several observations by saying that some particles (fermions) cannot share quantum numbers (Pauli's exclusion principle). If we look at the simplest atom with two electrons (helium) we can see that both electrons occupy the same orbital (i.e. energy level), so there must be another quantum number that distinguishes them, and we call that "spin".

It turns out that particles with spin actually behave similarly to macroscopic spinning bodies, having analogous properties to angular momentum, so it is a good name. 
 

Offline evan_au

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Re: How do I explain quantum spin for a lay person?
« Reply #4 on: 25/03/2016 20:27:24 »
Perhaps the best-known application of particle spin is Magnetic Resonance Imaging (MRI).

This relies on the fact that some atomic nuclei have a net spin, making them like little gyroscopes that respond to external magnetic and radio fields. Usually Hydrogen is used (present in water and soft tissues), although other odd-numbered atoms can also be used (with bigger magnets).

When lined up by radio waves and released, these nuclei circle around like unbalanced gyroscopes, with a characteristic frequency which depends on the magnetic field.

By having different magnetic fields in different parts of the MRI machine, these atoms emit different frequencies, which can all be picked up by a sensitive radio receiver, which then creates a map of the distribution of hydrogen in your body's soft tissues. This makes it a good complement to X-Rays, which are better at viewing bones.
 

Offline puppypower

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Re: How do I explain quantum spin for a lay person?
« Reply #5 on: 26/03/2016 12:19:35 »
Spin is linear motion with curvature. Both are similar in many ways, beyond the difference in geometry. Both spin and linear motion carry momentum. Spin has angular momentum, while linear motion has just momentum. Angular simply tells us the velocity has high curvature. If we had a rock on a string, and rotated this in a circle and let the string break, there is conservation of momentum.

In terms of electrons in atoms, opposite spin electrons will attract, while same spin electrons will repel in terms of their magnetic fields.  This is essentially the same as opposite direction currents in linear wires will attract, while same direction currents in linear wires will repel. The affect is consistent for both. 

Spin is both a velocity and an acceleration, while linear motion is just a velocity. What that means is spin contains extra time potential beyond the velocity of linear motion. This is inferred from acceleration having the units of d/t/t, while velocity is d/t. Spin has extra units of time. The acceleration implicit of spin contains extra time potential.

If you look at gravity and GR, GR curves space-time. GR can cause linear motion to curve by means of the mass following the curvature of space-time. Spin is essentially paralleling an affect analogous to paths confined by the curvature of space-time. If we had a collapsing cloud of space gas and debris, as it collapses, linear motion will begin to curve until rotation appears in the bulk mass, where all the linear paths of all the particles, take on circular paths; orbit.

The question becomes, where is the extra time potential, associated with spin; d/t/t versus the original d/t, coming from? When gravity lowers potential, if gravity is a force, then there should be an exothermic output as the gravitational potential between two particles of mass lower. This exothermic output does not appear to be in the form of photon type energy, since we don't see such an  energy signal, per se. Rather the exothermic output appears to be connected to potential in time. This would need to be seen in an indirect way or inferred from affect.

Much of the original linear momentum of the particles that make up the earth; originally moving toward center of gravity, are perpetuated in time as angular momentum; rotation of the earth. If we did not have the earth spinning, the original linear velocity and momentum of its atoms would have already stopped in time. The spin conserves the original momentum in time, with the extra time conserve in the acceleration implicit of spin.

One possible way to explain this in terms of the intrinsic spin of an electron, is to consider an electron. In larger atoms, like Gold, electrons move close enough to the speed of light to induce relativistic affects; relativistic quantum chemistry. These show up as anomalies in the Schrödinger equations used to model the  wave functions of atomic orbitals. These anomalies can be explain with accuracy using relativity. The yellow color of the metal Gold is connected to a time shift in the reflected light; all the reflected light gains a yellow cast. This is not due to excited electrons since even red reflected light will get a yellow cast, with yellow higher energy than red.

If we assume these gold electrons are moving at relativistic speeds, there will define a reference that is different from the bulk earth reference of the lab. Their time clocks will run slightly slower due to SR. This time difference has a connection to the Heisenberg uncertainty principle, since it is not easy to synchronize two clocks that run at different rates. One can synchronized time for an instant; position. Or one has to anticipate when the second hands from both clocks coincide; momentum. 

That aside, since the electron clock is running slower, than the lab clock, they will linger slightly longer in time; the electron twin ages slower. The extra potential in time is expressed, at elates in part, as the extra time in spin. Nuclei and the nucleons of atoms also have spin; time potential. Nuclei orbitals are less based on energy levels, as there are on time potential levels; spin based magnetic addition.
« Last Edit: 26/03/2016 12:31:29 by puppypower »
 

Offline chiralSPO

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Re: How do I explain quantum spin for a lay person?
« Reply #6 on: 26/03/2016 13:59:43 »
In larger atoms, like Gold, electrons move close enough to the speed of light to induce relativistic affects; relativistic quantum chemistry. These show up as anomalies in the Schrödinger equations used to model the  wave functions of atomic orbitals. These anomalies can be explain with accuracy using relativity. The yellow color of the metal Gold is connected to a time shift in the reflected light; all the reflected light gains a yellow cast. This is not due to excited electrons since even red reflected light will get a yellow cast, with yellow higher energy than red.


Yes, gold is heavy enough that relativistic effects cannot be ignored. But this (quoted above) is not an accurate explanation of the color of gold.

First, let me say that all atoms experience relativistic effects, not just gold. The heavier the atom is, the stronger the relativistic effect, so for all atoms like gold, mercury, radon, uranium etc. these effects must be taken into account for an accurate description of their electronic structures.

The color of gold is influenced by these effects insofar as the electronic structure is partially determined by the relativistic contribution, and it is the electronic structure of bulk gold that determines its color. If it were purely an issue of relativity causing the color, why isn't mercury colored? Also why would we be able to make red gold, green gold, white gold and purple gold by alloying the gold with other elements? The alloys have slightly different band gaps, and therefore absorb photons of different energies, and therefore appear different colors.

puppypower's "time shift" explanation is totally false. Gold appears red under red light (reflecting the red light back unchanged) and appears very dark under blue light (absorbing a significant amount of this light).

The electronic structure of gold and its colors is explained very well here: http://www.webexhibits.org/causesofcolor/9.html

Another topic they hit on that I will mention briefly is that of colorful gold nanoparticles. Old stained glass is colored using gold nanopartilces (they didn't know what nanoparticles were at the time, but they still made 'em and used em'). Depending on their size, gold NPs can appear red, green, blue, yellow, or pretty much any other color of the rainbow. This is due to plasmonic effects (essentially the NPs act as little conductive antenae, but instead of absorbing radio waves, they are tuned for EM radiation in the visible region--and the size of the particle determines the frequency that it absorbs at).
 

Offline JohnDuffield

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Re: How do I explain quantum spin for a lay person?
« Reply #7 on: 27/03/2016 16:25:47 »
I was asked today at a Q and A session by a member of the public to please explain what physicists mean when they talk about quantum spin. I'd be really interested in how everyone here would have approached this please?
This is a tricky one, in that we have hard scientific evidence like the Einstein-de Haas effect which "demonstrates that spin angular momentum is indeed of the same nature as the angular momentum of rotating bodies as conceived in classical mechanics". There's also things like electron magnetic moment and the way electrons and positrons move in a magnetic field that make it clear spin is real. That's what physicists used to think, see Goudsmith and The discovery of the electron spin. However physicists nowadays say quantum spin is nothing like classical spin. They'll refer to what agyejy said above about spinning faster than light. You can find an example of that in an old version of the Wikipedia Stern-Gerlach article:

"Even if the electron radius were as large as 2.8 fm (the classical electron radius), its surface would have to be rotating at 2.3×1011 m/s. The speed of rotation at the surface would be in excess of the speed of light, 2.998×108 m/s, and is thus impossible".

This says the electron can't be rotating like a planet so it can't be rotating at all. But it's a non-sequitur, because the electron is a spin ½ particle. It doesn't rotate like a planet. It has a wave nature, featuring a rotation you can see in the depiction of a spinor. There's a major-axis rotation AND a minor-axis rotation which together might be called a Dirac spinor or bispinor. The "and" operates as a multiplier. The electron has instrinsic spin like a cyclone has intrinsic spin. Take the spin away from the cyclone using an anticyclone, and all you've got is wind. Take the spin away from the electron using a positron, and all you've got left is light. But you can't stand up and talk about this because it isn't mainstream.   
 

Offline jeffreyH

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Re: How do I explain quantum spin for a lay person?
« Reply #8 on: 27/03/2016 16:36:38 »
Does that mean you are sitting down John?
 

Offline Tim the Plumber

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Re: How do I explain quantum spin for a lay person?
« Reply #9 on: 27/03/2016 22:12:29 »
Answer;

Sometimes physics is bad for your head. If you need it expalained don't go there. There is a whole load of weird head blowing stuff to get through before any answer will make any sense what so ever. So electrons have a characteristic we call spin but it's just something that electrons do.

If you can explain what difference the different spin things do then.......
 

Offline evan_au

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Re: How do I explain quantum spin for a lay person?
« Reply #10 on: 27/03/2016 23:07:17 »
A game that helps explain spin?
http://en.wikipedia.org/wiki/Tangloids

The game seems to be made of 2 hand-size blocks of wood, and 3 strings.
But from the description, I can't understand how it works.
Maybe a good toy for a science fair?
 

Offline jeffreyH

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Re: How do I explain quantum spin for a lay person?
« Reply #11 on: 27/03/2016 23:54:13 »
A game that helps explain spin?
http://en.wikipedia.org/wiki/Tangloids

The game seems to be made of 2 hand-size blocks of wood, and 3 strings.
But from the description, I can't understand how it works.
Maybe a good toy for a science fair?

That is actually very good. The clue for the solution is translation.
 

Offline PmbNEP

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Re: How do I explain quantum spin for a lay person?
« Reply #12 on: 30/03/2016 03:37:37 »
I was asked today at a Q and A session by a member of the public to please explain what physicists mean when they talk about quantum spin.

I'd be really interested in how everyone here would have approached this please?
This is something which is very simple to define. Answer: Spin is the z-component of an intrinsic angular momentum of a particle. That's actually the definition that you'd find in a quantum mechanics text (Which axis is depends on the naming convention used in an experiment).

From Wikipedia under Spin_(physics)
Quote
The most convenient quantum mechanical description of particle's spin is therefore with a set of complex numbers corresponding to amplitudes of finding a given value of projection of its intrinsic angular momentum on a given axis.
 

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Re: How do I explain quantum spin for a lay person?
« Reply #12 on: 30/03/2016 03:37:37 »

 

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